The use of a four-channel colour sensor to measure chlorophyll and carotenoid concentration

A self-contained 4-channel sub-surface color sensor built by the School of Ocean Sciences is tested for the use of measuring phytoplankton pigments. The four 10 nm wide wavebands are centered at 435 nm (blue), 485 nm (cyan), 565 nm (yellow) and 665 nm (red). The instrument can be used for estimating chlorophyll a concentration from changes of the ratio of upwelling irradiances in the cyan and yellow wavebands and has been successfully calibrated for optical case I waters, where phytoplankton play a key role in the absorption of light. However, the main absorbing pigments in the cyan waveband are not chlorophyll a but caroteniod pigments. These also show a good correlation using the same ratio. The strong relationship between chlorophyll a and carotenoid concentrations in the phytoplankton cells explains the apparent chlorophyll a-cyan:yellow relationship. Application to Case II waters is being examined.